policy: extend API version checks for cffi This is just a stub for future extension. I could add a version constant to CFFI modules by putting it to both ffi.set_source() and ffi.cdef(), but that doesn't seem right. So for now, cffi modules will be explicitly unversioned (i.e. version constant must be undefined or set to None.) We can revisit it later when we need to consider CFFI support more seriously.